1. Field of the Invention
The present invention relates to a developing device of an electrographic recording apparatus, a laser printer, and the like.
2. Description of the Related Art
The conventional electrophotographic apparatus adopts a magnetic brush development system, in which a developing agent (i.e., two-component developer) comprised of a toner and a carrier is used to develop electrostatic latent images formed on a photosensitive body (drum). The toner is consumed during this process, and thus fresh toner must be supplied periodically, and when the end of the predetermined service life of the carrier is reached, the used carrier must be replaced by fresh carrier. Accordingly, the used developing agent comprised of the carrier and the toner is removed (exhausted) from the developing device, and collected in a suitable container. During the removal of the developing agent, the surface of the photosensitive drum can be easily damaged, and this must be prevented.
A conventional developing device and the operation thereof are now explained with reference to FIG. 1.
As shown in FIG. 1, the developing device 2 is arranged adjacent to a photosensitive drum 1 and comprises a developing agent holding vessel 3, a toner holding vessel 4, a toner feeding roller 17, a developing roller 7, and a stirring roller 8. The toner and carrier (magnetic material powder) are mixed in the vessel 3 by the stirring roller 8, so that the toner and carrier are given electrostatic charges by friction and the charges are uniformly distributed, to form a developing agent. The developing roller (i.e., magnetic roller) 7 is composed of a rotatable non-magnetic sleeve 6 and a stationary magnetic roller 5 surrounded by the sleeve 6; an air gap being formed between the sleeve 6 and the roller 5. The magnetic roller 5 has several magnetic poles, for example, 5 poles, (three N-poles and two S-poles) and one of N-poles is arranged to face the photosensitive drum 1. The developing agent is fed to the developing roller 7 by the stirring roller 8 and is then adsorbed on the surface of the non-magnetic sleeve 6 due to the magnetic force thereof. The adsorbed agent is carried toward a developing zone by the rotation of the sleeve 6, in a direction A, and on the way to the developing zone, a blade 9 cuts the adsorbed developing agent to a predetermined uniform height and the cut (excess) agent flows through a flow regulating plate 10 and is returned to the developing agent holding vessel 3. When the developing agent reaches the developing zone, i.e., the position at which the magnetic pole is facing the photosensitive drum, the height of the developing agent is increased (i.e., a developing agent brush is formed) by the magnetic force, to bring the agent into contact with the surface of the drum. In addition, a developing bias voltage is applied between the non-magnetic sleeve 6 and the photosensitive drum 1, whereby the toner of the developing agent adheres to an electrostatic latent image formed on the drum 1 to form a developed image. After development, the developing agent is carried further by the rotation of the sleeve 6 and then allowed to drop down, due to a weak magnetic force portion of the developing roller 7, under its own weight and return to the vessel 3.
The toner is consumed during the movement thereof from the developing device 2 to the photosensitive drum 1, and to maintain a desired toner concentration, new toner is periodically fed from the toner vessel 4 through the toner feeding roller 17, but the carrier is continuously used in the developing device until the end of a predetermined service life thereof. When the developing agent containing the expired carrier is removed from the developing agent holding vessel 3, a lid 11 covering an opening 18 at a lower portion of the vessel 3 is opened and the stirring roller 8 and the developing roller 7 are rotated to cause the agent to flow out through the opening 18. In this case, the portion (i.e., magnetic brush) of the developing agent comes into pressurized contact with the surface of the photosensitive drum 1, which does not rotate, and thus the contact surface portion of the drum 1 is damaged. Therefore, to prevent this damage, the developing agent must be separated from the photosensitive drum 1. To accomplish this separation, the developing device 2 is moved away from the drum 1. For example, the device 2 is supported by a holding base 12 which can be swung about a fulcrum 13 with the aid of a cam 14. The base 12 is forced into contact with the cam 14 by a spring 15 and is swung by the rotation of the cam 14. The cam 14 is driven by operating a lever 16 by hand, to move the device 2. This moving mechanism complicates the structure of the developing device, and further, since the heavy developing device 2 (8-10 kg) must be manually moved by the lever 16, an increased labor force becomes necessary.